Such an energy converting apparatus is also referred to as a thermionic generator (TIG). The TIG can serve for instance as source of electrical energy. Conversion by means of this effect takes place for instance in compact TIGs for generating electrical energy in spacecraft, often in combination with nuclear generated heat. Conversely, the converting apparatus, in combination with a tunnel effect of electrons, can also pump heat by means of the Peltier effect. The converting apparatus can for instance be used as heat pump, for instance as cooling element in an air-conditioning system or a refrigerator.
A known converting apparatus of the above stated type comprises an electrode provided with an emitter and collector, with vacuum or an ionizable gas as medium present therebetween. In order to release from the surface of the electrode the electrons must first overcome a threshold voltage, the so-called work function of the electrode material. Because of the magnitude of the work function electrons are only released from the emitter at relatively high temperatures and are then carried to the collector since heat, in this case the kinetic energy of the electrons or ions, flows from the warm emitter to the colder collector. An electric current likewise begins to flow due to the electrical charge of the electrons. Because the thermionic effect is however only effective at temperatures above about 1600 K, much radiation is sent from the emitter to the collector and a relatively large amount of heat loss occurs. The maximum efficiency that is achieved is thus 10 to 12%, which is uneconomic for most applications. The application of the known apparatus therefore remains limited to space travel and to applications wherein a relatively low weight and long-term reliable availability are crucial factors.
In order to solve the problem of great heat loss U.S. Pat. No. 6,876,123-B2 provides a TIG wherein a plurality of electrodes are stacked on each other and held at a mutual distance by insulating elements for the purpose of forming a gap between the electrodes. If the gap is small enough, the electrons can also tunnel and the effective work function is decreased, so that the thermionic effect can also be applied effectively at low temperatures. At low temperatures however the gap must then be so small that the ratio between the dimensions of the electrodes and the gap height becomes relatively large, this up to 1:10,000,000. Thermal stresses can hereby occur, whereby the insulator elements can shift and come loose. The surfaces of the electrodes can also come into contact with each other, thereby terminating the operation of the known TIG. U.S. Pat. No. 6,876,123-B2 further makes use of a gap height of 5 to 10 nm. This is however too large to realize a tunnel effect with a high efficiency. When on the other hand the distance between the electrodes is smaller, it becomes more difficult to maintain this intermediate distance. The known apparatus also involves removal of materials from the gap between the electrodes. This is difficult to realize in the case of caps with a height of less than 5 nm and diameters or lengths of the electrodes in the order of centimeters.
With intermediate distances smaller than the original 5 nm the problems of thermal expansion and manufacture are also greater. In an electrode of a few centimeters the differences in expansion are many times greater than the height of the gap, in the order of 200 nm per degree, and the thermal stresses can become so great that the insulator elements are pressed into the electrode. The electrodes can thus nevertheless come into mutual contact, thereby terminating the operation of the TIG. The electrodes can also detach locally, whereby the tunnel effect at the location in question is no longer active and, as weakest link in the series, seriously limits the electric current in the whole stack. With a smaller gap height the heat conduction through the insulators is here greater, and more layers of electrodes are necessary to limit the thermal loss. In the apparatus of U.S. Pat. No. 6,876,123-B2 the thickness of the insulating layer or of the insulator elements is equal to the height of the gap and the insulating layer covers 25% of the electrode surface. Since the electrons also tunnel through the insulator elements, the electrons will tunnel less through the vacuum. The effective area of the vacuum part of the electrodes hereby becomes (much) smaller. The converting apparatus is thus largely a metal insulating metal (MIM) diode, for which it is the case that thousands of layers are necessary to limit thermal losses. The plurality of electrodes stacked in series are further roughly the same per layer, and the geometry and material type per layer is not adapted to the local temperature. The partial efficiencies per layer are greatly dependent on the temperature and the energy density, or rather the electric current density, this latter remaining roughly the same for all layers. The overall efficiency can hereby be greatly reduced.